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Chapter 10,Mapping Models to Relational Schema

Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 2

Mapping an Object Model to a Database• UML object models can be mapped to relational

databases:• Some degradation occurs because all UML constructs

must be mapped to a single relational database construct - the table

• Mapping of classes, attributes and associations• Each class is mapped to a table• Each class attribute is mapped onto a column in the

table• An instance of a class represents a row in the table• A many-to-many association is mapped into its own

table• A one-to-many association is implemented as buried

foreign key

• Methods are not mapped.


Mapping a Class to a Table

User

+firstName:String+login:String+email:String

id:long firstName:text[25] login:text[8] email:text[32]

User table


Primary and Foreign Keys

• Any set of attributes that could be used to uniquely identify any data record in a relational table is called a candidate key

• The actual candidate key that is used in the application to identify the records is called the primary key

• The primary key of a table is a set of attributes whose values uniquely identify the data records in the table

• A foreign key is an attribute (or a set of attributes) that references the primary key of another table.


Example for Primary and Foreign Keys

User table

Candidate key

login email

“am384” “[email protected]”

“js289” “[email protected]”

firstName

“alice”

“john”

“bd” “[email protected]”“bob”

Candidate key

Primary key

League table login

“am384”

“bd”

name

“tictactoeNovice”

“tictactoeExpert”

“js289”“chessNovice”

Foreign key referencing User table


Buried Association

LeagueLeagueOwner *1

id:long

LeagueOwner table

...owner:long

League table

...id:long

• Associations with multiplicity “one” can be implemented using a foreign key

For one-to-many associations we add the foreign key to the table representing the class on the “many” end

For all other associations we can select either class at the end of the association.

owner


Another Example for Buried Association

Transaction

transactionID

Portfolio

portfolioID...

*

portfolioID ...

Portfolio TableTransaction Table

transactionID portfolioID

Foreign Key


Mapping Many-To-Many Associations

City

cityName

AirportairportCodeairportName

* *Serves

cityNameHoustonAlbanyMunich

Hamburg

City Table

airportCodeIAHHOUALBMUCHAM

Airport Table

airportNameIntercontinental

HobbyAlbany CountyMunich Airport

Hamburg Airport

cityNameHoustonHoustonAlbanyMunich

Hamburg

Serves Table

airportCodeIAHHOUALBMUCHAM

In this case we need a separate table for the association

Separate table forthe association “Serves”

Primary KeyPrimary Key


Another Many-to-Many Association Mapping

PlayerTournament **

id

Tournament table

23

name ...

novice

24 experttournament player

TournamentPlayerAssociation table

23 56

23 79

Player table

id

56

name ...

alice

79 john

We need the Tournament/Player association as a separate table


Realizing Inheritance

• Relational databases do not support inheritance • Two possibilities to map an inheritance

association to a database schema• With a separate table (”vertical mapping”)

• The attributes of the superclass and the subclasses are mapped to different tables

• By duplicating columns (”horizontal mapping”)• There is no table for the superclass• Each subclass is mapped to a table containing the

attributes of the subclass and the attributes of the superclass


Realizing inheritance with a separate table (Vertical mapping)

User table

id

56

name ...

zoe

79 john

role

LeagueOwner

Player

Player

User

LeagueOwnermaxNumLeagues credits

name

Player table

id

79

credits ...

126

id

LeagueOwner table

56

maxNumLeagues ...

12


Realizing inheritance by duplicating columns (Horizontal Mapping)

Player

User

LeagueOwner

maxNumLeagues credits

name

id

LeagueOwner table

56

maxNumLeagues ...

12

name

zoe

Player table

id

79

credits ...

126

name

john


Comparison: Separate Tables vs Duplicated Columns

• The trade-off is between modifiability and response time

• How likely is a change of the superclass?• What are the performance requirements for queries?

• Separate table mapping (Vertical mapping)We can add attributes to the superclass easily by

adding a column to the superclass tableSearching for the attributes of an object requires a join

operation.

• Duplicated columns (Horizontal Mapping)Modifying the database schema is more complex and

error-proneIndividual objects are not fragmented across a number

of tables, resulting in faster queries


Summary

• Four mapping concepts: • Model transformation improves the compliance of the

object design model with a design goal• Forward engineering improves the consistency of the

code with respect to the object design model• Refactoring improves code readability/modifiability• Reverse engineering discovers the design from the code.

• Model transformations and forward engineering techniques:

• Optimizing the class model• Mapping associations to collections• Mapping contracts to exceptions• Mapping class model to storage schemas.

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Documents

superclass table

relational table

table aninstanceof

id leagueowner table

association separate

long leagueowner table

separate table mappingvertical

long league table